1. Field of the Invention
The present invention relates to a lead frame and a semiconductor device, and more particularly to a lead frame used for a resin-encapsulated type semiconductor device and a resin-encapsulated type semiconductor device.
2. Description of the Related Art
Heretofore, a resin-encapsulated semiconductor device or semiconductor package comprises, as shown in FIG. 4, a lead frame 50 having a plurality of leads 51 and a die pad 52 disposed at a position surrounded by top ends of the leads 51, a semiconductor element 54 mounted by way of a bonding agent 53 on the obverse of the die pad 52 and an encapsulation portion 55 for integrally resin-encapsulating the semiconductor element 54, the top ends of the leads 51 and the die pad 52.
Most of the lead frame 50 equipped with the semiconductor device is formed at present with an iron-42% nickel material (hereinafter simply referred to as a 42-alloy material) or a material mainly composed of copper (hereinafter simply referred to as a copper material), and development has proceeded for the copper material in view of its high heat conductivity, low cost and good workability.
However, in a lead frame made of a copper material, since oxide layers composed of copper oxides (CuO and Cu2O) are formed on the obverse and the reverse of the die pad after fabrication to worsen the adhesion between the bonding agent comprising, for example, an epoxy resin type silver paste and the obverse of the die pad and the adhesion between the encapsulating resin material and the reverse of the die pad. Therefore, a semiconductor device manufactured by using a lead frame made of the copper material involves a problem that cracks 56 are liable to be formed in the encapsulation portion as shown in FIG. 4 upon mounting the device by reflow to a substrate in a moistened state of the encapsulation portion to deteriorate cracking resistance, compared with a semiconductor device manufactured by using a lead frame made of the 42 alloy material.
That is, since the encapsulating resin is highly hygroscopic and the encapsulation portion of the semiconductor device is in a moistened state upon mounting, steam evolves in the encapsulation portion by high temperature during reflow. As a result, peeling occurs easily by the generation of steam at a boundary between the bonding agent and the die pad and at a boundary between the encapsulating resin material and the reverse of the die pad where the adhesion is poor and, further, cracks are caused in the encapsulation portion by the stresses formed by the expansion of the steams in the peeled portion.
The foregoing problem can be solved. according to the present invention by a lead frame comprising a plurality of leads and a die pad which are formed by using a copper material wherein, in the leads and the die pad, at least the obverse and the reverse of the die pad comprise a nickel plated surface.
The present invention also provides a semiconductor device comprising the lead frame as described above, a semiconductor element mounted by way of a bonding agent to the obverse of the die pad and a resin encapsulation portion for integrally resin-encapsulating the semiconductor element and the top ends of the leads.
The plated nickel has higher adhesion with a bonding agent such as a silver paste and a resin-encapsulating material, compared with copper oxides and, particularly, nickel oxide formed by oxidation of plated nickel has a good adhesion with the encapsulating resin material. Furthermore, the nickel plated surface has excellent adhesion with a solder used upon mounting.
In the lead frame according to the present invention, since at least the obverse and the reverse of the die pad comprises such a nickel plated surface, a higher adhesion strength is obtained between the obverse of the die pad and the bonding agent upon mounting the semiconductor element to the obverse of the die pad by way of the bonding agent. In addition, a high adhesion strength can be obtained between the reverse of the die pad and the encapsulating resin compared with the prior art in which copper oxides were formed to the obverse and the reverse of the die pad when the semiconductor element mounted on the obverse of the die pad and the top end of the lead are resin-encapsulated. Further, when plated nickel at the reverse of the die pad is oxidized by being exposed to air during resin encapsulation, the reverse of the die pad and the encapsulating resin are adhered more strongly. Furthermore, when the leads are also formed with the nickel plated surface together with the die pad, high adhesion can be obtained between the leads and the solder when the semiconductor device constituted by using the lead frame is mounted on a substrate by means of solder.
In the semiconductor device according to the present invention, since the lead frame as described above of the present invention is used, the reverse of the die pad and the encapsulating resin are more strongly adhered compared with the semiconductor device of the prior art constituted by using the lead frame only consisting of the copper material. Further, together with the die pad, if the leads are also formed with the nickel plated surface, the leads can be soldered with good adhesion upon mounting the semiconductor device to the substrate by using a solder.